A hard disk, or hard disk drive (HDD), is a data storage device that uses magnetic storage to store and retrieve digital information. It consists of one or more spinning disks coated with a magnetic material, where data is written and read by a moving arm. Hard disks are commonly used in computers, laptops, and servers to store operating systems, applications, and files. Hard disks are known for their large storage capacity and cost-effectiveness compared to other storage options like solid-state drives (SSDs). However, they are generally slower and more prone to mechanical failure due to their moving parts. Despite this, they remain a popular choice for bulk data storage in many devices.